The leukemic cell of chronic lymphocytic leukemia patients with autoimmune hemolitic anemia produce isotype-switched immunoglobulins that are preferentially encoded by the 51p1 and DP-50 VH genes

The observed skewing in the repertoire of VH, D, and JH gene segments strongly indicates that the leukemic Igs in CLL patients with AIHA have been selected because of particular antigen specificities that could promote the development of the hemolytic anemia. This could occur either by a direct involvement of the CLL Ig in erythrocyte destruction or, indirectly, by inducing the production of pathogenic antierythrocyte antibodies. The first possibility was supported by our recent finding that in most CLL cases a small subset of leukemic cells undergoes Ig isotype-switching.9 The isotypeswitched CLL cells were found to produce both membrane and secretory -chain transcripts, of which the latter could encode part of the IgG antibodies that have been implicated in RBC destruction. To test this hypothesis, we investigated the reactivity of the Igs expressed by the leukemic cells from the CLL-AIHA patients HA-2 and HA-5. These two Igs contained strikingly similar heavy-chain CDR3 regions that were associated with the 51p1 VH gene segment; both of them used λ light chains that belonged to different Vλ gene families (HA-2VL is 95.5% homologous to Humlv 122 from the VλI family, and HA-5VL is 93% homologous to Humlv318 from the VλIII family). The VH and VL region genes from these two Igs were expressed in eukaryotic cell lines as dimeric single-chain (sc) Fv fragments, and their reactivities were tested against erythrocytes and a panel of self and foreign antigens. Indirect immunofluorescence analysis with the affinity-purified scFv fragments showed the absence of any significant RBC binding. On the other hand, both antibody fragments showed similar low-affinity binding to human IgG (FIGURE 1B) and various reactivities against other antigens such as phosphorylcholine, dextran, lipopolysaccharide, and single-stranded DNA. These data indicate that the 51pl-encoded CLL Igs are not directly involved in RBC destruction, but could promote the development of AIHA by inducing a disturbance in the idiotypic network that would ultimately lead to the expansion of autoreactive B cell clones with antierythrocyte specificity. Alternatively, secreted isotype-switched CLL Igs could bind to anti-RBC antibodies and could promote the adherence of sensitized red blood cells to the Fc receptors on macrophages.